1. Field of the Invention
The present invention relates to an electric-mechanical-acoustic-transducer for generating a mechanical vibration or a sound from an electric signal, and a portable communication device including the electric-mechanical-acoustic-transducer.
2. Description of the Related Art
Conventionally, a cellular phone includes both a sound generator for generating a bell sound or a melody, and a micromotor for generating a mechanical vibration, as means for informing the user of a call arrival. The conventional cellular phone further needs a sound receiving speaker (i.e., a receiver) for reproducing a received sound.
In order to reduce the size and weight, and also to reduce the number of components, of a portable communication device such as a cellular phone, a mechanism for providing two functions of sound generation and mechanical vibration generation with one electric-mechanical-acoustic-transducer is proposed (Japanese Laid-Open Utility Model Publication No. 5-85192).
FIG. 10 is a cross-sectional view of such a conventional electric-mechanical-acoustic-transducer 5000. As shown in FIG. 10, the electric-mechanical-acoustic-transducer 5000 includes a circular diaphragm 1. An outer periphery of the diaphragm 1 is attached to a case 2. The case 2 includes a bottom plate 5, and a yoke 3 is attached to the bottom plate 5. A suspension 6 is supported by the case 2, and a magnet 4 is supported by the suspension 6.
A voice coil 7 is inserted into a magnetic gap formed between an inner circumferential surface of the yoke 3 and an outer circumferential surface of the magnet 4. One end of the voice coil 7 is fixed to the diaphragm 1. The yoke 3 and the magnet 4 are included in a magnetic circuit, and the suspension 6 and the magnet 4 are included in a mechanical vibration system.
The electric-mechanical-acoustic-transducer 5000 having the above-described structure operates as follows. When an electric signal is applied to the voice coil 7, action-reaction forces act on the voice coil 7 and the magnetic circuit. Assuming that an action force acts on the voice coil 7, the action force vibrates the diaphragm 1, to which the voice coil 7 is attached, and thus a sound is generated.
A reaction force acting on the magnetic circuit vibrates the magnet 4 supported by the suspension 6. The vibration is conveyed to the case 2 via the suspension 6. Thus, the case 2 is vibrated.
The electric-mechanical-acoustic-transducer 5000 has the following problem.
The suspension 6 is formed of a material having a small internal loss, such as, for example, a leaf spring. Therefore, the sharpness (Q factor) of the mechanical vibration at a resonance frequency is increased. FIG. 11 shows a frequency characteristic of a mechanical vibration force generated by a vibration of the magnet 4 with a chain line 300.
Where the resonance frequency of the mechanical vibration system is f0, and frequencies at which a mechanical vibration force having a value lower by 3 dB than the value of the mechanical vibration force at the resonance frequency f0 are f1 and f2, the sharpness (Q factor) Qf0 is represented by expression (1).Qf0=f0/(f2−f1)  (1)
In the electric-mechanical-acoustic-transducer 5000, the sharpness (Q factor) of the mechanical vibration force at the resonance frequency of the mechanical vibration system is large. Therefore, the resonance frequency of the mechanical vibration system changes in accordance with a change in use conditions of a cellular phone (e.g., the way the user holds the cellular phone or the way the user positions the cellular phone). When the frequency of an input signal and the resonance frequency are deviated from each other even slightly, the mechanical vibration force is decreased to the extent that a sufficient vibration is not provided. In order to solve this problem, it is conceivable to additionally provide a circuit for constantly tracking the resonance frequency and thus changing the frequency of the input signal. Such a circuit undesirably enlarges the scale of the system.